Lake Kleifarvatn is the largest lake of the Reykjanes peninsula.  It is the third largest lake of southern Iceland, 9, 1 km², and one of the deepest lakes of the country, 97 m.  Its catchment area is small and it has a very limited discharge on the surface.  Consequently it varies in size during the year.  Since the year 2000 it has diminished in area because of two major earthquakes, which probably opened up fissures at its bottom.  Char fries from Lake Hlidarvatn were released into the lake in the sixties and have thrived quite well.  The best catch can be expected in the southernmost part of the lake where hot water from some hot springs runs into the lake.  Elsewhere the lake is very cold.  There is a small fishing lodge by the lake.  The barren, volcanic surroundings of the lake are unique and beautiful.  According to legends, a monster in the shape of a worm and size of a medium sized whale occupies the lake.

In the wake of the earthquakes (June 17 and 21) in the Southwestern Lowlands in 2000 (6,5 Richter), the lake started shrinking because of increased leakage through new fissures.  In August 2003, one major earthquake (5,4 Richter) and hundreds of smaller ones for days afterwards, shook the area, but did not do much damage.